Due to gradually reduced size and integrated multiple functions of electronic products, semiconductor packages are accordingly required high integration and miniaturization such as multi chip module packages that have become the mainstream technology in the market. Accordingly, multilayer circuit boards, on which active and passive components are mounted, also need to be formed with high density wiring patterns and interlayer connections to match high density integrated circuit.
FIG. 1 is a cross-sectional view of a prior art multilayer circuit board. As shown in FIG. 1, first, a plurality of through holes 11a is formed in a core board 11. Then, the core board 11 is plated to form conductive layers on both sides of the core board 11 and inner walls of the through holes 11a. The conductive layer on both sides of the core board 11 is subsequently patterned. As a result, circuit layers 12a and 12b are formed on both sides of the core board 11 and plated through holes 12c are formed inside the through holes 11a to electrically connect the circuit layers 12a and 12b. Build-up circuit structures 13 are formed respectively on the circuit layers 12a and 12b, which are electrically connected with each other through the plated through holes 12c. In this way, a multilayer circuit board 10 is formed.
Since the build-up circuit structures 13 on both sides of the core board 11 are electrically connected with each other through the plated through holes 12c, a core board is needed to be disposed between the build-up circuit structures 13, thereby increasing the thickness of the circuit board.
In addition, to form through holes in the core board, high cost drilling machines are needed, thereby increasing the fabrication cost. In general, there are two ways to form through holes: mechanical drilling and laser drilling. To form through holes by mechanical drilling, fine drilling bits are needed, which are easy to wear out and accordingly need to be changed periodically, thereby increasing the material cost. On the other hand, it is difficult to drill through the core board by laser drilling, especially to drill through the core board having multiple metal layers. Moreover, to effectively form conductive layer on inner walls of the plated through holes 12c, diameter of the through holes can not be too small, thereby increasing the difficulty of forming fine pitch high density package.
Moreover, long conductive path between the build-up circuit structures 13 on both sides of the core board 11 that are electrically connected through the plated through holes 12c and parasitic capacitance and inductance generated by long plated through holes introduce large impedance to the circuit board, which affects signal integrity especially of high frequency circuit. Further, the electrical performance of the circuit board is affected.
Furthermore, build-up circuit structures 13 are symmetrically formed on both sides of the core board 11, which sometimes introduce some of redundant circuit layers, thereby increasing the fabrication cost and also increasing the difficulty of forming thin circuit board.
Accordingly, there is a need to provide a thin circuit board, which can overcome the above drawbacks.